Robust and reliable trace-gas analyzers (e.g., for isotopic carbon, methane, nitroxide, ammonia, etc.), suitable for long-term measurements, are not commercially available. Such analyzers are needed to achieve a better understanding of the global budget of these trace gases and to quantify their exchanges between the atmosphere and the terrestrial biosphere. This project will develop a tunable diode laser-based open-path methane analyzer using newly-developed laser technology. The new analyzer will be suitable for deployment over a wide range of ecosystems to understand methane exchange between the atmosphere and the surface. In Phase I, an open-path methane analyzer, with a newly available vertical cavity surface emitting laser (VCSEL) and wavelength modulation spectroscopy (WMS), was developed. The resolution of the analyzer was shown to be better than 10 ppb at a 10-Hz sampling rate, exceeding the required specifications for methane flux measurement; therefore, even low methane fluxes can be measured using eddy covariance with this instrument. Phase II will further develop the open-path methane analyzer by adding a multiple-pass Herriott optical cell to the VCSEL and WMS. Instrument-embedded software and application software also will be developed. The analyzer will be capable of field deployment for methane flux measurement over various ecosystems and other surface contexts (e.g. landfills and animal husbandry lots). Commercial Applications and other Benefits as described by the awardee: The lack of robust and reliable methane analyzers has limited observations of methane exchange, between terrestrial ecosystems and the atmosphere, to short-time intervals only. The new analyzer should allow measurements of methane exchange to be made year-round, with a minor maintenance requirement. In addition, the hardware and signal processing software developed for the analyzer should provide the basis for developing other trace gas and isotopic element analyzers, including but not limited to analyzers for ammonia, nitroxide, carbon isotope, and oxygen isotope